In a semiconductor device in which an analog circuit and a digital circuit are mounted in a mixed manner, there is a problem in which a clock signal used in the digital circuit reaches the analog circuit through a substrate, becomes noise and gives a bad effect to an operation of the analog circuit. As a method for suppressing such cross talk through the substrate, a method is known that a contact is connected to a region between the circuits in the semiconductor substrate and the semiconductor substrate is grounded through this contact so as to stabilize the potential. However, with this method, since the semiconductor substrate is still present between the circuits, cross talk through the substrate cannot be effectively suppressed.
Also, a technique in which an SOI (silicon on insulator) substrate is used as a semiconductor substrate has been proposed. The SOI substrate has an embedded insulating film made of a silicon oxide provided on a base material, and a thin silicon layer is provided on this embedded insulating film. By forming an element in the silicon layer of the SOT substrate and by surrounding the periphery of the element by an element isolation insulating film, this element can be fully insulated from the periphery, and cross talk through the substrate can be effectively suppressed. However, the element is surrounded by a silicon oxide in this type of a semiconductor device, and since heat conductivity of a silicon oxide is much lower than the heat conductivity of silicon, radiation performance is extremely low. As a result, the temperature rises due to self heat generation of the element, and the driving current thereof is decreased. The heat conductivity of silicon (Si) is 1.56 W/cm·K, while the heat conductivity of silicon oxide (SiO2) is 0.014 W/cm·K.
In order to improve radiation performance of the SOI substrate, use of an SON (silicon on nothing) substrate is one idea. The SON substrate is a silicon substrate in which a cavity is formed inside. By embedding an insulating material such as a silicon oxide or the like in the cavity of the SON substrate, an embedded insulating film can be locally formed in the silicon substrate. Then, by forming an element on this locally formed embedded insulating film, cross talk is suppressed to some degree, while radiation performance can be made higher than that of the SOI substrate. However, even in the SON substrate, since the embedded insulating film having low heat conductivity is present below the element, the radiation performance is not sufficient.